The invention relates to a travelling wave tube (TWT). The invention more specifically relates to a travelling wave tube with coupled cavities in which the beam is focused by a system of permanent magnets of the periodic permanent magnet type (PPM).
Such a focusing system of the so-called "alternating permanent magnet type" is constituted, as in the prior art, by rings pressed against one another around the axis of the tube and consisting alternately of magnets and pole pieces. In this case the pole pieces made from soft iron or some other material with a low reluctance are incorporated into copper cavity walls, so as to conduct the magnetic flux as near as possible to the electron beam.
As the thermal conductivity of soft iron is approximately 6 times lower than that of copper, the thermal resistance between the beam and the tube cooling circuit is at least 6 times higher when the pole faces are incorporated into the cavity walls. If it is also accepted that the current intercepted by the delay line is, all things being equal, 2 to 4 times higher in the case of focusing by permanent magnets than in the case of focusing by a solenoid, the continuous power which it is possible to apply to the tube and consequently the maximum high frequency power available at the output of such a tube is significantly lower than in the case of focusing by a solenoid. These high frequencies correspond to decimeter or centimeter waves, varying from a few gigahertz to more than 10 gigahertz. Towards the high frequency output of the tube the increase in the beam diameter can be such that, under the influence more particularly of the high frequency field of the cavities, the openings thereof melt under the impact of the intercepted electrons. This limits the power applied to the tube.
However, in spite of this the technology of travelling wave tubes is often directed towards the solution using permanent magnets for focusing the beam due to the convenience which it provides, more particularly from the standpoint of fitting focusing members and of the operation of the tube, due to the elimination of the continuous source necessary for supplying the solenoid.